Instead of being defined by specific types, each category of iterator is defined by the operations that can be performed on it. This definition means that any type that supports the necessary operations can be used as an iterator -- for example, a pointer supports all of the operations required by RandomAccessIterator, so a pointer can be used anywhere a RandomAccessIterator is expected.

All of the iterator categories (except OutputIterator) can be organized into a hierarchy, where more powerful iterator categories (e.g. RandomAccessIterator) support the operations of less powerful categories (e.g. InputIterator). If an iterator falls into one of these categories and also satisfies the requirements of OutputIterator, then it is called a mutable iterator and supports both input and output. Non-mutable iterators are called constant iterators.